1. Field of the Invention
The present invention relates to an ink evaluation method for evaluating the stability of ink jet, an ink jet unit for discharging ink in the form of ink droplets and ink used in the ink jet unit.
2. Discussion of the Background
Ink jet units include, for example, an ink jet unit mounted on an ink jet printer. The ink jet unit is basically composed of pressure chambers, from which ink is discharged, the nozzles disposed within the pressure chamber for discharging the ink, and a drive means for discharging the ink in the pressure chamber as ink droplets. Note that the ink is supplied to the pressure chamber from an ink tank through an ink feeding passage, in the middle of which the ink is filtered.
A wide variety of techniques for stably discharging the ink from the nozzles have been proposed with respect to this type of ink jet unit. One of them, for example, is such a technique to enable the ink meeting a given requirement for any pressure loss possibly incurring during passing through the filter to be applied to the ink jet unit (Refer to JP-A No. 90210/1995).
Ink types to be supplied to the ink jet unit include aqueous ink, oil ink, solvent ink, and Ultra Violet ink (UV cure ink) (hereafter, simply referred to as UV ink). The aqueous and oil inks are often used when the ink is applied to water-absorbing objects, while the solvent and UV ink are often used when they are applied to non-water-absorbing objects.
Since the solvent ink has a high volatility index, the nozzles may be clogged due to solvent evaporation to dryness at a high frequency. On the contrary, since the UV ink has a very low volatility index, it may scarcely be clogged due to solvent evaporation to dryness. This type of UV ink gets cured by photo-curing reaction with a UV ray. This means that the reaction between a photopolymerization initiator contained in the UV ink and a monomer or oligomer is induced by a UV ray to form a highly polymerized compound, resulting in the cured UV ink. In addition to this property, the UV ink has such another property that it tends to get cured in a short period of time, for example within one second after being discharged, preventing an organic solvent contained it from evaporating. Further, the UV ink is excellent in abrasion resistance than other types of ink. Owing to these advantages, a demand for UV-ink ink jet units is increasingly growing.
On the other hand, in any types of ink such as the UV ink, in which a pigment is dispersed, the pigment tends to easily agglutinate, leading to possible malfunction of discharging. This may cause a problem of deteriorated stability of ink discharging performance. Thus, the ink with a higher stability index of ink discharging performance is desired and it is a criterion for ink evaluation whether the ink discharging stability index is acceptable.
Accordingly, an object of the present invention is to provide an ink evaluation method, which allows the preferred ink with a high discharging performance to be selected out of various types of ink.
Another object of the present invention is to provide the ink jet unit capable of discharging stably the ink from the nozzles and the ink suitable for the unit.
These objects of the present invention can be achieved by a novel ink evaluation method, ink, and ink jet unit of the present invention.
The new ink evaluation method of the present invention, therefore, comprises: a first measurement step of making a given amount of solution pass through a filter and measuring the time for the solution to pass through the filter (filter passing time); a second measurement step of making pass through the filter the ink having a solid material dispersed in a solvent and having the same viscosity and the same amount as those of the solution, and measuring the time for the ink to pass through the filter (filter passing time); a calculation step of calculating a ratio of the filter passing times between the solution and the ink; and a determination step of determining whether the ink discharging stability index is acceptable based on the measured ratio.
As known from the description above, the novel ink of the present invention is the ink determined to have a higher ink discharging stability index based on the novel ink evaluation method of the present invention.
Thus, the novel ink discharging unit of the present invention is configured so that the ink determined to have a higher ink discharging stability index based on the novel ink evaluation method of the present invention may be contained in it and discharged as ink droplets.